Driveline Angles 101

This YouTube video demonstrates (in the best high school shop class way) how driveline angles work. You can see and hear what happens as the angles change at the u-joints. What you hear are the changes in speed as the driveshaft rotates in an elliptical pattern rather than spherical and this causes a vibration in the seats, mirrors, gearshift levers, and other parts. It can also damage or destroy driveshaft u-joints and other parts like the clutch, tailshaft housings, and rear axle bearings.

Now that you understand how driveline angles work and why it's important to measure them, let's move on to how you measure driveline angles.

This free app from Tremec for smart phones can be of great help. Just follow the instructions given in the app and the app will even tell you which angle needs to be fixed. Otherwise, you'll need to buy an angle gauge at a local tool store and follow the instructions below. We strongly suggest using a digital angle finder or a smart phone to do this. An analog angle finder is very difficult to read with the numbers so close together and Spicer even recommends rounding the angles to the nearest quarter degree, so analog angle finders seem to be insufficient for the task.

With the weight on the wheels, all tires inflated to their normal operating pressure, and with the car parked on a relatively flat surface, place your angle finder on the rear face of the engine block, perpendicular to the driveline. This is your "zero;" you can zero your digital readers if you're using a digital gauge. The bellhousing in this example was an SFI approved, welded steel bellhousing that covered our rear face of block, but the front flange is just a quarter inch thick, flat piece of steel, so we were able to get our measurement there.

Move your gauge to the driveshaft and measure the angle; this is your first driveshaft-operating angle. Because the digital protractor was rotated 90 degrees from our "zero", we have to subtract 88.0 from 90, so we have 2.0 degrees, which is inside our 2-4 degree window.

After noting your angle, if you're using a digital reader, "zero" the reader again.

Move your gauge to the bearing cap on the rear axle (the flat surface that bolts to the driveshaft) and measure the angle (you may have to unbolt the driveshaft). This will give you your 2nd driveshaft-operating angle. Again, we've rotated our protractor 90 degrees, so we have to subtract 87.4 from 90 and that leaves our measurement at 2.6 degrees.

You may want to repeat the above with a load (for instance: passengers) in the car.

TIP: If you used an analog or digital protractor that is incapable of resetting the zero measurement, you will find this tool from Spicer's website very handy. "Up" would mean what you're measuring rises from the front to the rear of the vehicle and "Down" would mean the component descends from front to rear.

TIP: Do not over torque the bolts on the U-bolt clamps or you risk distorting the bearing cap leading to premature bearing failure. U-bolts for 1310 and 1330 joints are to be tightened to 17 ftlbs.

Ideally, both driveshaft-operating angles will be 1 to 3 degrees. Anything over 3 degrees at the rear axle will shorten your universal joint's life and could create vibration. If your angle is larger than 3 degrees, you must make sure not to exceed max driveshaft RPM as shown on this Spicer table.

Engines are almost universally tilted down between 2-4 degrees and because your transmission is directly bolted to the engine, it too is between 2-4 degrees. When you're changing a component of your drivetrain, like the transmission, it's important to maintain this factory angle. The transmission crossmember plays a vital role in establishing this first angle. A one size fits all crossmember fits about as well as one size fits all clothing, which is to say, not at all. The result is often an angle that is out of spec. We have a separate crossmember for almost every application because we've already calculated the exact position the tailhousing will need to rest to maintain an acceptable driveline angle. This may mean you'll have to cut your tunnel. Sacrificing driveline angles to save your tunnel will cause you more problems in the long run.

If your first driveshaft-operating angle is out of spec, you may need to put in spacers or modify your crossmember. If you end up modifying your crossmember, you'll want to make sure it is still sturdy; a solidly built crossmember is vital in the event of a crash.

If your second driveshaft-operating angle is out more than 1-3 degrees, you'll need to fix it. You can change your pinion angle or if you have a leaf spring suspension, you can buy leaf spring pinion angle shims like the ones here and correct the angle. Even factory rear axles can droop over time and may require you to correct the angle.

If you're building a completely custom job and would like to take a graduate course in driveline angles, we'd direct you to this Driveline Angles 501 course courtesy of Inland Empire Driveline in California.